The use of antihalo dyes for light sensitive elements is well known. The most common application is for silver halide based films coated on a transparent supporting medium. Light passing through the sensitive layer can be internally reflected from the back surface of the support, and re-illuminates the sensitive layer some distance laterally from the original point of exposure. This exposure is readily visible as a "halo" if the image is that of a bright point source.
The most common uses of antihalo dyes are in the form of a backing, coated on the support. This is destroyed during processing, either mechanically or chemically. In an alternative construction a dye may be coated between the sensitive layer and the support, and destroyed chemically during processing. A further construction involves the use of grey-dyed base as used in many fast negative camera films. Dyes have also been used as underlayers, below the sensitive layers, in X-ray films, where a sensitive layer is coated on each side of the support. The dye prevents radiation passing through one sensitive layer and exposing the layer on the opposite side. Such additional exposure is known to give rise to greatly decreased sharpness, even though the image of a bright point source would not be rendered as a distinct halo.
The degree of halation or light spread is particularly troublesome in all of the materials mentioned above since the light traverses several times the thickness of the transparent base before re-exposing the layer. Therefore, in all of the above described applications the antihalo dye is intended to absorb substantially all of the unwanted radiation, although some benefit is to be gained by partial absorption. The decrease in exposure, and therefore of the apparent sensitivity of the material, caused by the presence of the antihalo dye is seen to be a small disadvantage compared with the advantage of increased image sharpness.
The uses of antihalo dyes so far described are applied to coatings on substantially transparent base. In the case of a transparent layer of refractive material such as gelatin or other polymer coated on a diffusely reflecting opaque support, light striking the support is reflected at all angles within the transparent medium. Although some light escapes from the surface a considerable fraction, usually over 50%, is totally reflected at the gelatin or polymer/air interface. This light re-illuminates the base in a manner analogous to that described above. The light from the base gives rise to a succession of multiple reflections between the base and the polymer/air interface.
When the polymer layer contains a light sensitive element, the multiple reflections are, to some extent, limited by absorption of light within the layer. However, many sensitive materials are sufficiently transparent to the exposing radiation that multiple reflections are still possible. It follows that the light sensitive material receives multiple exposures from successive passages of the light across the layer, much of the exposure taking place at distances from the original point of entry that are large compared with the layer thickness. The result is that the sensitive layer displays higher apparent sensitivity but lower image sharpness than the same formulation coated on a non-reflecting support such as a backed film. Simple geometrical optics leads to the conclusion that, if exposure can be expressed as the integration of light flux density multiplied by the distance travelled within the layer, a totally transparent non-scattering material will receive over nine times the exposure received by the material on an absorbing support.
The distance between the polymer-air interface and the reflecting base is usually much less than the thickness of a transparent base; furthermore the image sharpness is usually much lower than that demanded from films coated on transparent base, and therefore much of the exposure arising from the internal reflections can be utilised for little or no apparent loss of image sharpness. The degree to which the loss of image sharpness can be tolerated depends on the particular application. Therefore antihalo layers applied to materials coated on reflecting base should preferably be only partially absorbing.
Examples of light sensitive materials comprising a reflective support bearing a substantially transparent light sensitive medium are photothermographic materials, particularly those commonly referred to as "dry silver systems".
Dry silver systems which comprise a thermally developable photosensitive mixture of light sensitive silver halide with a silver salt of an organic fatty acid, e.g. behenic acid, are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075. Known dry silver systems have employed antihalation and/or acutance dyes in order to ensure a sharp image, the antihalo dyes being stable under the manufacture and storage conditions of dry silver but readily bleachable during or after the heat development step. The antihalation layers containing the bleachable dyes have been positioned between the support and photosensitive layer(s), between photosensitive layers and, in the case when a transparent support has been used, on the side of the support opposite the photosensitive layers(s).
Known dyestuffs and processes suitable for antihalo applications in dry silver systems include thermally bleachable dyes as disclosed in U.S. Pat. Nos. 3,745,009, 4,033,948, 4,088,497, 4,153,463 and 4,283,487; photobleachable o-nitroarylidene dyes as disclosed in U.S. Pat. No. 4,028,113; and thermochromic dyes as disclosed in U.S. Pat. No. 3,769,019.
British Patent Specification No. 1 588 097 discloses heat bleachable compositions for use in photography comprising a benzopinacol which forms ketyl radicals on heating to a temperature above 100.degree. C. and a dye which is bleached by ketyl radicals. The heat bleachable compositions are useful in photothermographic systems as antihalation layers when coated between the support and a light sensitive layer or on the back of a transparent support. The compositions may also be used in photothermographic systems as a light screening layer coated directly on top of the light sensitive layers or between two light sensitive layers in order to completely absorb light of unwanted wavelength but remain transparent to light of the desired wavelength.
British Patent Application No. GB 2 004 380A discloses heat bleachable compositions for use in photography comprising a hexaaryl biimidazole and a dye which is bleached upon reaction with a product formed on heating the hexaaryl biimidazole. The compositions are useful as antihalation and filter layers in a variety of photographic materials. For antihalation purposes, they may usefully be in a layer (or group of adjacent layers) between the photosensitive layer and the support, in the support itself or, if the support is transparent, in a layer (or group of adjacent layers) coated on the side of the support opposite to that carrying the photosensitive layer.
Heretofore the use of antihalo dyes in photothermographic systems has been limited to their presence in an antihalation underlayer positioned beneath the light sensitive medium, an antihalation layer positioned between two light sensitive layers, an antihalation backside coating on a transparent support and the presence of acutance dyes in one or more layers constituting the light sensitive medium.
British Patent Specification No. 2 054 184B discloses an example of an antihalo dye deliberately intended to give partial absorption. This Patent discloses a photographic material sensitive to visible radiation and having at least one silver halide emulsion layer, the uppermost emulsion layer having an antihalation coating which has an absorption maximum in the spectral range in which said uppermost emulsion layer has its maximum sensitivity and which has an optical density of at least 0.10 at the said absorption maximum. Light which is scattered back towards the top surface of the layer can be totally internally reflected and re-enter the sensitive layer, producing additional loss of image sharpness. The effect is greatest when the sensitive layer is below several others which are substantially transparent to the radiation in question. By coating a dye in the surface of the material the light that is internally reflected is attenuated, giving a useful interchange between image sharpness and sensitivity. Because the dye is above the sensitive layer, it must be traversed by the exposing radiation before any of it reaches the sensitive layer. Therefore, it must only cause partial absorption in order to be useful.
The necessity for the additional layer arises from the use of a sensitive medium sufficiently scattering to produce back reflection from within the layer carrying the medium. Thus, the use of an antihalo dye behind the sensitive layer would be necessary but not sufficient for the materials described. The case of photothermographic elements coated on a reflecting base is different in that substantially no back-scatter from the sensitive medium occurs and therefore only an antihalo layer is required to quench multiple reflections between the base and polymer-air interface.
The presence of a heat bleachable topcoat antihalation layer on photothermographic elements has not been used or considered desirable in the prior art on the assumption that the presence of such a topcoat in sufficient quantity to increase image sharpness would significantly reduce sensitivity of the material, and be less efficient compared to the use of antihalation underlayers and backside layers.
It has now been found that the presence of a bleachable antihalation topcoat on photothermographic elements may provide a balance of sensitivity and image sharpness which surprisingly is comparable to and often superior to that obtained using acutance dyes or antihalation underlayers.